The invention relates to a method of manufacturing a master for the fabrication of projection screens, the screens comprising a substrate of transparent material provided on one side with a lens structure consisting of a pattern of parallel rectilinear lens elements, which are separated by grooves. According to the method a pattern of parallel rectilinear grooves and ribs corresponding to the lens structure is formed on one side of a plate of machinable material by a tooling operation, the separate grooves being formed by a slotting operation performed by means of a profile chisel. The invention further relates to a tool for carrying out the method.
For the manufacture of projection screens, more particularly of rear projection screens, in general, first a so-called master or mother mould is manufactured; then a number of production moulds are manufactured from this master by means of known electrodeposition techniques, via one or more intermediate moulds. By means of the production moulds thus obtained, the projection screens can be manufactured with the aid of various forming processes, such as pressing, injection-moulding, etc. For this purpose, however, in general the known replica techniques are used.
It is known from U.S. Pat. No. 4,573,764 to provide the rib pattern during the manufacture of a master by means of a diamond chisel. By means of such a generally digitally controlled one-point chisel, the grooves and ribs are formed by so-called countouring, that is to say that each rib is profiled in a large number of small steps. This method of profiling is time-consuming. The procedure of contouring a master having for instance the dimensions of 0.8.times.1.0 m takes several days. During this period, the chisel must be replaced several times because of wear. The process can be controlled only with difficulty in view of the risk of temperature fluctuations, vibrations and the like, which may occur during this long operation and to which the work-piece, the tool and the precision arrangement on which the operation is carried out are subjected. The lens structure must be designed so that the pattern of grooves and ribs is accessible throughout its depth to the cutting point of the chisel, for example by providing dead spaces which in themselves are undesirable and which impose limits on the design of the lens structure.
These disadvantages are obviated in the method mentioned in the opening paragraph. In this method known from U.S. Pat. No. 3,765,281, a profile chisel is used for the manufacture of the master. The cutting profile of the chisel is formed in accordance with the desired cross-section of the grooves and ribs. By the use of a profile chisel, the grooves are not formed by contouring, but by means of a cutting operation called slotting and by the profile of the chisel. The time required for the manufacture of a master is reduced from several days to several hours. Since the manufacture can take place in a comparatively short time, the process can be more readily controlled.
However, this known method cannot be used universally and is not suitable for the manufacture of a master whose ribs have a thin wall portion and/or a sharp edge. During the step of cutting a groove the rib being formed, which is bounded on one side by the groove being cut and on the other side by the adjacent groove already cut, is loaded unilaterally. As a result, there is a risk of undesired deformation of the rib, more particularly of lateral bending of the rib towards the groove already cut. Deformation of the ribs results in an inability to achieve the accuracy requirements imposed and the process is not reproducible.
The invention has for its object to provide a method which does not suffer from these limitations and is suitable for the manufacture of masters with different patterns and profiles of grooves and ribs.